You'll end up with some really boring matches where people run away for 10 minutes or both fighters gas, start throwing arm punches and can't hurt the other person. This already happens in many matches but the time runs out and the judges score it.

Its quite rare to see matches in the lower levels go the two extension rounds here, the ruleset pretty much makes you go for the knockdown and not care about points. Maybe go one 5 minute round with it automatically made a draw if it goes the distance?

"Boxing is the art of hitting an opponent from the furthest distance away, exposing the least amount of your body while getting into position to punch with maximum leverage and not getting hit."
Kenny Weldon

Bad idea, you either have the guys gas to the point where they cannot KO each other.....

Or you have everyone KO'ing each other all day.

Full Contact Karate is brutal on the body IMO, and lengthening the time of the match will only add more injuries, making for a bad experience for cometitors and the likelihood of people having to drop out of the tourney sky rocket.